A non-linear element, for example a power amplifier, causes distortion to a signal. Reduction of distortion caused by amplifiers has been attempted by various means. Because the distortion of amplitude or phase as a function of amplitude of an input signal often resembles a known function, distortion has been compensated for with a component having a similar characteristic behaviour. Typical components for predistortion include a diode, a field effect transistor or a bipolar junction transistor. Although this solution is simple, it is also inaccurate. A characteristic curve of one component cannot well enough cancel the distortion of a non-linear element, such as a power amplifier.
Predistortion can be performed by using look-up tables, which can, in addition, be updated in order to achieve adaptability since amplifier distortion is affected by temperature, age of the amplifier and changes of the signal fed to the amplifier, for example.
Instead of look-up tables, a polynomial higher than the first order can be used to estimate distortion. Typically, the order has to be at least five or even seven for cancelling the distortion well enough. This, however, increases the number of multiplication operations drastically.
Look-up tables and polynomials result in very complicated and non-ideal compensation circuits which cause problematic delays in signal processing. Furthermore, multiplication operators used in a polynomial solution are difficult to implement and cause unnecessary delay. Irrespective of whether or not a linearization is used, power amplifiers cannot, thus, linearly amplify a signal if the power level of the incoming signal varies, which is the case e.g. in UMTS (Universal Mobile Telephone System), CDMA (Code Division Multiple Access) and WCDMA (Wide-band CDMA) radio systems.